الفهرس 
THEORETICAL ANALYSISOnly 14 pages are availabe for public view
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Abstract
The present work aimed at the study of the influence of the
internal stresses on the creep behaviour of the austenitic
steel X8 CrNiMoNb 16 16.
The experimental work included different creep experiments
under different stresses and temperatures in the primary,
secondary (steady state),
and tertiary stages. The strain
transient dip test was applied to determine the level of the
internal stress during different creep stages.
The
theoretical part included the application of the
dislocation theory to deduce certain relations to predict the
level of the internal stresses and to describe the creep
behaviour of the austenitic steel.
The following results have been obtained:
The creep behaviour of the austenitic steel X8 CrNiMoNb 16 16
is highly affected by the internal back stresses which resist
the creep deformation.
- The internal stress is developed with the initial creep
loading and progresses as the deformation continues.
- The internal stress -which is proportional to the square root
of the immobile dislocation density~ increases during the
primary stage with the creep strain approaching its steady
state value.
- The effective stress -which is the difference between the applied and the internal stress- decreases as the creep
strain increases causing the creep rate to decrease during
the primary stage.
- A relationship was deduced to predict the creep rate during
the primary stage as a function of the steady state creep
rate, the steady state internal stress, and the instantaneous
internal stress. The predictions of this relationship showed
reasonable agreement with the experimental creep rate data.
- The steady state creep rate was found to depend on the steady
state effective stress by a power law.
- The steady state internal stress depends on the structure,
the applied stress, and the creep temperature.
- As the applied stress increases, the steady state internal
stress increases till a saturation value occurs at higher
stresses. This saturatiuon value depends on the temperature.
- A relation was found to describe the dependence of this
saturation value with the exponential of the reciprocal of
the absolute temperature.
- An empirical relation was applied to predict the steady state
internal stress value under different temperatures or
stresses.
- A power law was found to control the effective stress
dependence of the tertiary creep rate at different times in
the tertiary stage.
- The predictions of these relationships enabled the prediction
of the creep rates after stress increments or decrements
during primary, secondary, and tertiary stages.